A scanning system used to produce video data signals of the above type typically comprises a light source for illuminating the medium on which the images are printed and a scanner device including a plurality of sensing devices each adapted to receive light reflected from a respective discrete portion of the surface of the medium and to generate video data signals representative of the density of the element of the image printed on that discrete portion of the surface of the medium. The scanning device may be a single row of sensing devices which is moved across the medium in order to scan over the whole of the medium. The analog video data signal output of each sensing device is converted into a digital signal and represents on a grey scale the density of the element of the image printed on the discrete portion of the medium from which light is received by that sensing device.
Since there may be variations in the level of illumination of each portion of the surface of the medium and variations in the sensitivity of each sensing device and in the value of its output when it receives no illumination and inaccuracies in any associated optical system, it is necessary to correct the output video data signal generated in each sensing device in order that the set of video data signals produced by the scanning system accurately represents the image printed on the medium.
In order to correct the video data signal generated in each sensing device it is known to use the values of the signals generated when sensing light in two reference conditions corresponding respectively to the absence of any printed image element and to the presence of a densely printed image element on the medium in order to derive two reference quantities. When subsequently scanning a printed medium, the video data signal produced for each discrete portion of the medium is compared with these two reference quantities and a corrected value for the signal obtained. The corrected value is a digital signal. This correction operation ensures that all the video data signals from the sensing devices accurately represent the elements of the image scanned.
The digitization and correction operation utilizes an analog to digital converter and comparator unit. This unit is supplied with two reference quantities establishing the maximum and minimum values of a range of values. This range is divided into a number of discrete levels. An input analog signal to the unit is compared with these discrete levels and the corresponding level determined. A digital signal representing the corresponding level is the output from the unit. The number of bits in the digital signal defines the number of discrete levels in the range. The difference between adjacent levels, which defines the accuracy of the output signal, is dependent on the number of levels and the length of the range. For a given range the accuracy is increased by increasing the number of bits. However analog to digital converters of this type become increasingly more expensive and complicated to manufacture as the number of bits in the output signal increases.
In order to produce corrected values for video data signals, two reference quantities as described above are supplied to a unit of the above type to define the range with which the video data signals are compared. The corrected values are produced in the form of digital output signals.
It is convenient to use the same unit to establish the reference quantities. For efficient operation of the system it is necessary for the reference quantities to be established very accurately. However as indicated above achieving this accuracy by increasing the number of bits in the digital output signal is expensive and complicated.
The object of the present invention is to provide an improved analog video data signal digitization and correction system of the above type which utilizes an analog to digital converter operating with a relatively small number of bits while still producing the digital reference quantities with a relatively high accuracy.